Telomere dysfunction-induced foci (TIF) are detected in senescent human fibroblasts and murine organs, and elderly primates and have recently been used as a marker for aging. Currently, TIF is determined by the colocalization of telomeric DNA or proteins with phosphorylated histone H2AX (gamma-H2AX) using immunofluorescence or immuno-telomere FISH analysis. However, due to the incapability of these conventional assays to detect TIFs at short telomeres, it is unclear if eroded telomeres in human conditions, such as aging and age-related diseases become dysfunctional, thereby contributing to DNA damage signaling and cellular senescence. We are in the process to develop an assay to determine the colocalization of gamma-H2AX and telomeric DNA (or TIFs) in human and murine cells harboring critically short telomeres and telomeric DNA strand breaks.